This invention relates to self-twist yarns and to improved methods and apparatuses for making such yarns.
Self-twisted yarns are yarns formed by twisting a strand to provide it with alternating sections of S-twist and Z-twist. The alternately twisted strand is unstable in the sense that, if set free, it would untwist to release the torque energy it received from the alternate twisting step. When two such strands are in parallel adjacent relationship with sections of corresponding twist disposed side-by-side, the strands are released to untwist and entwine about each other to form a self-twisted plied yarn. A plurality of such yarns may then be twisted together by a similar technique to form a "cable". For purposes of this description, the term "strand" is used in its generic sense to include, inter alia, singles strands, plied yarns and cabled yarns. The strand may be a continuous bundle of filaments, a continuous form of discontinuous filament, a drafted carded sliver which is untreated or pretreated to increase its tensile strength, continuous filaments produced by a tow treatment process or a combination of staple fibers and one or more continuous filaments.
Background information regarding self-twisted yarns is found in the book "Self-Twist Yarn" by D. E. Henshaw, Merrow Publishing Company Ltd., Watford, Herts, England, 1971. A partial listing of U.S. Pat. Nos. relating to this technology is as follows:
Re 27,717--Breen et al PA1 4,074,511--Chambley & Norris PA1 3,225,533--Henshaw PA1 3,306,023--Henshaw et al PA1 3,353,344--Clendening, Jr. PA1 3,434,275--Backer et al PA1 3,488,939--Walls PA1 3,507,108--Yoshimura et al PA1 3,537,251--Kimura et al PA1 3,717,988--Walls PA1 3,775,955--Shah PA1 3,940,917--Strachan PA1 4,055,039--Movshovich et al PA1 4,068,459--Movshovich et al PA1 4,084,400--Movshovich et al.
Some of the prior self-twisting systems are continuous in the sense that the strand is twisted and plied while moving at a constant velocity. It is believed that at least some continuous systems are suitable for industrial practice, one such system being shown in the Chambley and Norris U.S. Pat. No. 4,074,511 which uses stationary reversible fluid vortex devices for giving the strand an alternate twist, and a device which moves at strand velocity for joining the twist reversal points or "nodes" of adjacent strands before they are released to self-twist together. This requires precise timing and phasing controls which are complex, impose speed limitations on the system and may be extremely sensitive, especially when the operation of the twister must be phased to correspond to a preexisting twist condition in the strand being processed.
Prior self-twist systems raise potential problems with respect to twist distribution and the frequency of the twist reversal nodes. Regarding twist distribution, there is a tendency for twist to back up behind the reversible twister so that when the twister direction is changed, a surge of opposite twist is released which must be leveled and redistributed before a satisfactory product may be made by self-twisting. This problem is addressed in the Chambley et al patent listed above of which I am a co-inventor. With respect to the frequency of the nodes, it should be mentioned that the nodes present a visual discontinuity in the yarn since they are substantially without twist. It is therefore desirable to have the twist reversal nodes spaced as far apart as possible. Existing systems limit the distance between nodes to the practical distance along which twist can be propagated from a twist insertion location. Twice this distance is attainable according to the present invention.